In the field of LSI, there have recently been widely and variously investigated the use of an interlayer dielectric film which is characterized in that it has a low dielectric constant (k) on the order of not more than 2.5 and which is used in combination with a copper wiring pattern or copper electrical connections. It has been proposed that the relative dielectric constant of the interlayer dielectric film is further reduced by making, porous, an oxide film having a low dielectric constant and serving as such an interlayer dielectric film. However, it has presently been known that the use of such porous film suffers from a variety of problems such that (1) the mechanical strength of the film is abruptly reduced, (2) the moisture included in the air is absorbed in and adsorbed onto the wall of the holes or pores present therein and (3) the presence of hydrophobic groups such as CH3 groups introduced into the film for the prevention of such moisture uptake would reduce the adhesion of the film to another film brought into contact with the same. For this reason, in the process for practically applying the porous film to a semiconductor device, in particular, in the chemical mechanical polishing (CMP) process for forming a Cu dual damascene wiring or electrical connection structure and the wire-bonding process, there are observed, for instance, the following problems: (1) the breakage of the porous film due to the reduction of the mechanical strength of the film, (2) the increase in the relative dielectric constant due to the moisture uptake and (3) the occurrence of peeling of the laminated film from the porous interlayer dielectric film due to the reduction of the adhesion between them, and these problems would become a major obstacle in putting the porous thin film into practical use.
Moreover, there has also been proposed a method for forming an oxide having uniform meso fine pores, which makes use of the self-organization of an organic compound and an inorganic compound, for instance, a method for forming such an oxide according to the hydrothermal synthesis technique in a sealed heat-resistant container using, for instance, silica gel and a surfactant (see, for instance, Patent Document 1 specified below) and it has recently been reported that such an oxide having uniform meso fine pores is formed into a film-like shape in order to use the same as a material for semiconductor devices.
For instance, there has been known a method for forming a porous silica film, which comprises the step of immersing a substrate into a liquid sol comprising a condensate of alkoxysilanes and a surfactant to thus precipitate the porous silica onto the substrate surface in the form of a film (see, for instance, Non-Patent Document 1 specified below). There has also been known a method for forming a film on a substrate, which comprises the steps of applying, onto the substrate surface, a solution obtained by blending an organic solvent with a condensate of alkoxysilanes and a surfactant, and then removing the organic solvent through evaporation (see, for instance, Non-Patent Document 2 specified below).
The method for precipitating porous silica onto the substrate surface as disclosed in the aforementioned Non-Patent Document 1 suffers from problems such that it takes a long period of time for the preparation of a film-like porous silica, that the majority of porous silica is precipitated in the form of powder and that the yield of the method is considerably low. For this reason, the method disclosed in Non-Patent Document 2 in which the organic solvent is removed through evaporation is rather effective for the preparation of a porous silica film as compared with the method disclosed in Non-Patent Document 1.
In the method disclosed in Non-Patent Document 2 in which a film is formed on a substrate by the removal of the organic solvent through evaporation, there has been proposed the use of organic solvents such as polyhydric alcohols, glycol ether solvents, glycol acetate ether solvents, amide type solvents, ketone type solvents, and carboxylic acid ester solvents (see, for instance, Patent Document 2 specified below).
However, the porous thin film prepared by the foregoing conventional method includes a large number of hydrophilic portions or sites within the holes or pores thereof, the thin film is thus liable to take in water vapor present in the air and this accordingly results in an increase of the relative dielectric constant of the porous thin film. For this reason, there has been proposed, as a method for the introduction of hydrophobic functional groups into the interlayer dielectric film, for instance, a method for maintaining the dielectric properties thereof by the prevention of any moisture uptake through the conversion of the silanol groups within the fine pores into trimethylsilyl groups (see, for instance, Patent Document 3 specified below). Nevertheless, it has been known that this method never permits the complete conversion of the silanol groups within the fine pores into trimethylsilyl groups (see, for instance, Non-Patent Document 3 specified below).
In addition, there has likewise been proposed a method for the production of a porous thin film which is free of any crack-propagation through the incorporation of Group IA or IIA element into the thin film (see, for instance, Patent Document 4 specified below). In this case, the concentration of such an element to be incorporated into the thin film ranges from 0.0001 to 0.015 parts by mass per 100 parts by mass of a hydrolyzed condensate and this Patent Document also states that if the amount thereof exceeds 0.015 parts by mass, the resulting solution is insufficient in the uniform applicability. However, the results of the supplementary examinations carried out by the inventors of this invention clearly indicate that the use of such an element in such a low amount leads to the formation of a thin film having insufficient mechanical strength and a rather high relative dielectric constant. For this reason, this method never permits the formation of a porous thin film having not only a low dielectric constant, but also a high mechanical strength.
Moreover, there has also been proposed a method for modifying the properties of a porous film which comprises the step of bringing a porous film mainly comprising Si—O bonds into close contact with a specific organic silicon atom-containing compound under heating without using any metallic catalyst in order to simultaneously improve the hydrophobicity and mechanical strength thereof (see, for instance, Patent Document 5 specified below). However, the resulting porous thin film has still been required for the further improvement of the mechanical strength thereof in order to put it into practical use.
Moreover, it has been known that the use of Pt as a catalyst can promote the reaction of siloxane (see, for instance, Patent Document 6 specified below). In this case, the technique relates to the method for making heat resistant ceramics hydrophobic and the resulting porous thin film essentially differs from that serving as an interlayer dielectric film for use in semiconductor devices in that the former contains various impurities including Na.
Patent Document 1: WO 91/11390 pamphlet (see, for instance, the contents of Claims);
Non-Patent Document 1: Nature, 1996, 379:703;
Non-Patent Document 2: Supramolecular Science, 1998, 5:247 or the like;
Patent Document 2: Japanese Un-Examined Patent Publication 2000-38509 (see, for instance, the contents of Section Nos. 0013 and 0014);
Patent Document 3: the specification of U.S. Pat. No. 6,208,014 (see, for instance, the contents of Claims and Abstract);
Non-Patent Document 3: J. Phys. Chem., B, 1997, 101:6525 or the like;
Patent Document 4: Japanese Un-Examined Patent Publication 2002-3784 (see, for instance, the contents of Claims and Section No. 0004);
Patent Document 5: Japanese Un-Examined Patent Publication 2004-292304 (see, for instance, the content of Claims and Section No. 0020);
Patent Document 6: the specification of U.S. Pat. No. 5,939,141.